Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Sterilizers are used in the medical, food and packaging industries to kill and thereby prevent the transmission of transmissible agents such as spores, fungi, and bacteria. A typical sterilizer creates a set of physical conditions in a sterilisation chamber that effectively kills nearly all of these transmissible agents.
Contacting articles in need of sterilisation with sterilant aerosols is one known method of sterilisation. A conventional aerosol sterilisation apparatus has a sterilisation chamber with an aerosol inlet valve and an aerosol outlet valve, an aerosol generator (typically an ultrasonic nebulizer) in fluid communication with the chamber via the inlet valve and a fan upstream of, and in fluid communication with, the aerosol generator.
In use, an article requiring sterilisation is placed in the chamber, which is then sealed. The aerosol inlet valve is opened and the aerosol outlet valve is closed. The fan is engaged, which creates a gas stream through or the past the aerosol generator into the chamber. A passive vent in the sterilisation chamber allows for pressure equalization as required, to permit gas flow in and out of the sterilisation chamber. The aerosol generator, which contains the desired sterilant, is then activated, putting a large number of small sterilant droplets into gas stream. The droplets are carried by the gas stream to create an aerosol which travels into the sterilisation chamber. The sterilant concentration in the aerosol stream can be adjusted by changing either the flow rate of the gas stream, the productivity of the aerosol generator, or the concentration of the liquid sterilant used.
The passive waste vent allows some flow to pass through it, allowing the sterilisation chamber to remain at approximately room pressure. This passive system may include a pathway for flow to the outside air past catalytic elements that react with the sterilant and break the sterilant down into a safer chemistry suitable for disposal.
After a period of time, the fan and the aerosol generator are deactivated and the air inlet valve is closed, hence completing the sterilant delivery phase. The exit valve is then opened and aerosol is actively removed, typically by way of a pump that pulls aerosol and vapour out of the sterilisation chamber at a high rate. The removal system may include a pathway for flow between the sterilisation chamber and outside air past catalytic elements that react with the sterilant and break the sterilant down into a safer chemistry suitable for disposal. The passive vent allows a source of fresh air to be drawn into the sterilisation chamber from the outside air.
It is generally desirable for the total sterilisation cycle time to be as short as possible. Short reprocessing durations increases the number of times the sterilised article can be used in a given period, which in turn increases the number of patients per day that can be treated. In the case where the article to be sterilised is a high-cost medical device, short cycle times can generate significant financial savings for a health care provider.
One of the limitations of using an aerosol-based sterilizer is that in order to gain the required level of microbiological reduction in a short sterilisation time a high concentration (ie a high mist density) of aerosol sterilant is required. During sterilisation, a high concentration of aerosol sterilant causes droplets to coalesce on the surface of the article. This can be particularly prevalent at a location on the article that is subject to a direct mist stream from the chamber inlet. This can also lead to multilayer B.E.T.-like absorption on the surface of the sterilized article. Coalesced and absorbed droplets can be difficult to remove from the article at the end of the sterilisation process. Large levels of residual sterilant left on the sterilised article can be harmful to operators and patients and as such are undesirable in a fully automated sterilisation device.
While the residual sterilant may be removed by washing, this is an expensive feature to add to an automated sterilisation device, and requires sterile water and fresh water supplies that cannot always be easily obtained. Alternatively, it is also undesirable to have staff hand-washing articles, as this requires the use of safety apparatus which can be expensive (such as fume hoods), can take up valuable time and space and moreover increases the risk of harmful sterilant coming into contact with an operator or patient.
A washing phase also requires a subsequent drying phase which adds considerably to apparatus turn-around times.
In conventional sterilization apparatus, the aerosol is usually introduced into the sterilization chamber at a single point, via a single chamber inlet port. As a result, the distribution of the aerosol particles tends to fan out from that single point. More droplets contact the article to be sterilised at a point close to the aerosol inlet port, and contact the article at higher velocity, leading to splattering on the surface and the build up of condensate. Similarly, the areas of the article to be sterilised which are more remote from the aerosol inlet may receive a smaller dose of aerosol. In such cases, in order to ensure sterilization of the entire article, it becomes necessary to increase the total sterilant dose to compensate for areas of the article that may receive a smaller dose. Increasing sterilant dose may be achieved by increasing the length of time to carry out the sterilisation or by increasing the amount of sterilant delivered in a given time. Both methods can exacerbate the splattering and condensation effect in areas close to the single chamber inlet port.
One method to reduce the level of condensation and splattering near the inlet port is to move the article to be sterilized further away from the inlet port, allowing it to better disperse before contacting the article. However, greater distances require larger sterilization chambers, and this is undesirable for a number of reasons. Due to space limitations in many medical healthcare facilities, it is desirable for sterilisers to be as small as possible while still being capable of housing the article to be sterilized. Small sterilization chambers are also advantageous because they are both faster to fill with sterilant and faster to remediate than larger chambers. However, a small sterilization chamber increases the difficulty of introducing aerosol into the chamber while having it contact the article in an evenly-distributed fashion.
Maintaining an even mist distribution inside a sterilization chamber is important to ensure that there is even sterilization of the article to be sterilized. Once introduced into the sterilization chamber, aerosol droplets tend to fall due to gravity which results in a greater mist concentration at the bottom of the chamber than at the top of the chamber. In order to maintain an even distribution top to bottom, a high aerosol flow rate can be used to provide droplet lift. In this case the gas stream moves in an upward direction at a faster rate than droplets fall. A downside of using such a method is that the gas stream velocities used result in greater velocities for smaller droplets, and as there is typically a wide range of droplet sizes in an aerosol it is difficult to optimise such a system. Additionally, the smaller and higher-velocity droplets can collide with the article to coalesce on its surface, thus making removal of residual sterilant difficult.
Using a dense mist is desirable, as it provides fast sterilization, which in turn can enable short sterilization cycles. However, in practice, dense mists are susceptible to condensation. Prior art sterilizers often require noisy, large and expensive apparatus to remove condensation in a time-effective manner. Thus, in prior art sterilizers, in order to avoid condensation, the density of mist needs to be limited, meaning that short sterilization times cannot be realized.
Accordingly, there is a need to find improved methods of delivery of the aerosol to a sterilisation chamber, particularly a small chamber, so that the aerosol is delivered to the article to be sterilised in an even manner and at a relatively low velocity to minimise the possibility of condensation.